patch 8.2.0241: crash when setting 'buftype' to "quickfix"

Problem:    Crash when setting 'buftype' to "quickfix".
Solution:   Check that error list is not NULL. (closes #5613)

src/testdir/test_quickfix.vim

@@ -1628,6 +1628,13 @@ func Test_setqflist_invalid_nr()
   eval []->setqflist(' ', {'nr' : $XXX_DOES_NOT_EXIST})
 endfunc
 
+func Test_setqflist_user_sets_buftype()
+  call setqflist([{'text': 'foo'}, {'text': 'bar'}])
+  set buftype=quickfix
+  call setqflist([], 'a')
+  enew
+endfunc
+
 func Test_quickfix_set_list_with_act()
   call XquickfixSetListWithAct('c')
   call XquickfixSetListWithAct('l')